President Barack Obama warned at a press conference on Monday that if the allegations that the Internal Revenue Service targeted conservative groups are proven true, then it will have to be held accountable.

"If in fact IRS personnel engaged in the kind of practices that have been reported on and were intentionally targeting conservative groups, then that's outrageous. And there's no place for it," Obama said during a joint news conference with British Prime Minister David Cameron.

"And they have to be held fully accountable. Because the IRS as an independent agency requires absolute integrity, and people have to have confidence that they're ... applying the laws in a nonpartisan way," he added, according to NBC News.

The IRS apologized on Friday for singling out organizations carrying the words "tea party" or "patriot" in their exemption applications between 2010 and 2012. Lois G. Lerner, the director of the Exempt Organizations Division of the IRS, admitted during a conference call that IRS agents made "mistakes" in targeting such groups for additional reviews during that period.

"This is truly outrageous and it contributes to the profound distrust that the American people have in government," Republican Sen. Susan Collins from Maine told CNN on Sunday, before Obama's remarks on Monday. "It is absolutely chilling that the IRS was singling out conservative groups for extra review, and I think it's very disappointing that the president hasn't personally condemned this and spoken out."

The IRS also apparently focused on applicants for the 501(c)(4) status that criticized "how the country is run" and wanted to educate the public on how to "make America a better place to live."

"These actions by the IRS are an outrageous abuse of power and a breach of the public's trust. Targeting groups based on their political views is not only inappropriate but it is intolerable," added Senate Finance Committee Chairman Max Baucus (D-Mont.), who called for a full investigation into the matter.

"We need to know who knew what, and exactly what mistakes were made. The American people have questions for the IRS and I intend to get answers. I want to review the Inspector General's report first, but the IRS should be prepared for a full investigation into this matter by the Senate Finance Committee. The IRS will now be the ones put under additional scrutiny."

Senate Republican leader Mitch McConnell of Kentucky also called for an investigation, describing the alleged IRS targeting as "thuggish practices."

"I call on the White House to conduct a transparent, government-wide review aimed at assuring the American people that these thuggish practices are not under way at the IRS or elsewhere in the administration against anyone, regardless of their political views," McConnell said.

At the press conference on Monday, Obama promised that actions will be taken to get to the bottom of what happened.

"I've got no patience with it," the president said, revealing that he found out about the IRS allegations through news reports on Friday. "I will not tolerate it and we will make sure that we find out exactly what happened on this."